The reduction of nitric oxide emissions of a combustion engine that works with oxygen surplus, in particular a diesel combustion engine, can take place with the aid of the so-called selective-catalytic-reduction technology (SCR). A reduction of nitric oxides into nitrogen and hydrogen is thereby carried out, whereby gaseous ammoniac or ammoniac in aqueous solution or urea in aqueous solution are used as reducing agent. The urea serves thereby as ammoniac carrier. With the aid of the metering system in front of a hydrolysis catalytic converter the reducing agent is injected into the exhaust gas pipe of the combustion engine. It is converted in the hydrolysis catalytic converter with the aid of the hydrolysis into ammoniac, which then further reduces the nitric oxides in the exhaust gas in the main SCR-catalytic converter, which is also called DENOX-catalytic converter. The main components of such a NOx-reduction system are a reducing agent tank, a pump, a pressure regulator, a pressure sensor and a metering valve. The pump advances the reducing agent that is stored in the reducing agent tank to the metering valve, with which the reducing agent is injected into the exhaust gas current upstream of the hydrolysis catalytic converter. The metering valve is thereby controlled by signals of a control unit, for example the control unit of the combustion engine, in order to supply a specific actually required amount of reducing agent. Substances that are present in aqueous solution and releasing ammoniac, such as urea, are preferably used, because they store reducing agent significantly easier and are easier to handle than for example gaseous ammoniac. Furthermore the ability to advance and meter this solution is technically significantly easier than for example the advancing and metering of gaseous substances. A disadvantage of the aqueous solution of for example urea is that the reducing agent solution has a freezing point of −11° C. Therefore devices have to be provided for heating the reducing agent in the reducing agent tank.
Because the NOx-reduction systems are an exhaust gas relevant part, failures, for example leaks and such alike, have to be detected. Previous leakage detection procedures evaluate the pressure in the SCR-systems either statistically or dynamically for the leakage detection and compare this pressure with absolute, constant default threshold values. In order to exclude here erroneous diagnoses every maximally possible tolerance has to be considered. Thereby the detection acuity is reduced in such a way that only bigger leakages are detected. Such a leakage detection qualifies basically only as “pipe tear off detection”, thus as a procedure for detecting whether a pipe interruption has taken place. It is extremely problematic at the previous procedure to detect an open clamped metering valve.